1. Field of the Invention
The present invention concerns a magnetic field generating device for an electron spin resonance system (hereinafter referred to as "ESR system") and, more particularly, it relates to such type of magnetic field generating device that is capable of continuously varying a magnetic field between the magnetic poles of permanent magnets arranged opposite to each other leaving a desired space therebetween while the magnetic field is made to keep a high degree of uniformity.
The ESR system is generally used in the field of research and development.
Since the ESR system can detect unpaired electrons due to natural radiation damages of the test objects and measure the ages of archaelogical remains, geology, minerals and fossil remains, it has also been utilized recently in the field of archaelogy and earth science.
For the magnetic field generating device forming an essential part of the ESR system, it is necessary to form a magnetic field of about 3000-4000 gausses at high accuracy and uniformity in the space in which a test object is arranged and to continuously change the intensity of the magnetic field within the range of .+-. about 100 gausses.
2. Description of the Prior Art
In the magnetic field generating device used for ESR system, electromagnets have usually been employed. It is easy to generate and continuously change a magnetic field in a desired space by continuously varying a current applied to the electromagnets but this involves a drawback that the entire device becomes large-sized and expensive.
Further, there has been proposed a structure using only permanent magnets as a magnetic field generating source so that the magnetic field intensity is changed by changing the opposing distance (length of the space) between the permanent magnets.
However, this structure has also had the disadvantage that when the opposing distance is changed, the degree of uniformity of the magnetic field is impaired due to various causes including the deviation of the axis of one of the opposing permanent magnets from that of the other.
The degree of uniformity of the magnetic field required in the ESR system is as high as less than 0.01% and, accordingly, there has not been known a device using a magnetic circuit comprising permanent magnets.
In view of the above, the present applicant has already proposed a magnetic field generating device for an ESR system in which a movable yoke 10 is disposed to one of a pair of stationary yokes 1a and 1b for continuously varying the distance between the opposing surfaces of the stationary yokes, thereby enabling the magnetic resistance of a magnetic path A formed by permanent magnets 3a, 3b, stationary yokes 1a and 1b, movable yoke 10 and a fixed yoke 8 (refer to Japanese Utility Model Laid-Open Hei 1-104574).
In FIG. 15, a space 2 is formed between a pair of permanent magnets 3a and 3b. Magnetic pole pieces 4a and 4b are disposed respectively to the opposing surfaces of the permanent magnets 3a and 3b.
Supports 5a, 5b and 5c are disposed for forming and maintaining a pair of stationary yokes 1a and 1b with a predetermined space and they are usually made of non-magnetic material. A gap 9 is formed between the movable yoke 10 and the fixed yoke 8.
In the constitution as described above, if the opposing distance between the movable yoke 10 and the fixed yoke 8 is made large, the magnetic resistance in the magnetic path A is increased to reduce the magnetic field intensity in the space 2. On the other hand, if the opposing distance is reduced, the magnetic resistance in the magnetic path A is decreased to increase the magnetic field intensity in the space 2.
Provision of a magnetic field generating device for an ESR system reduced in size and cost has been enabled by the proposed device. However, in the above-mentioned constitution, since the change of the magnetic field is great relative to the movement of the movable yoke, no fine change of the magnetic field can be obtained. Thus, the correspondence between the number of rotation for the movable yoke and the change of the magnetic field is not accurate, which is not sufficient for practicing the measurement of a resonance field at high accuracy and various improvements have still been required.